1. Field of the Invention
This invention relates to a grazing incidence spectrometer using a diffraction grating.
2. Description of the Prior Art
In the grazing incidence spectrometer according to the prior art, as shown in FIG. 1 of the accompanying drawings, an entrance slit 2 and a concave (spherical) diffraction grating 3 are disposed on a Rowland circle 1. At this time, the image plane is formed arcuately on the Rowland circle 1. Therefore, an exit slit movable on the image plne has been provided and a photoelectric detector has been integrally provided rearwardly of the exit slit and by moving the exit slit and the photoelectric detector on the image plane, scanning in wavelength has been effected to derive information for each wavelength. For the same reason, a photographic dry-plate has been disposed while being bent along the image plane (the Rowland circle) and by sensitizing this, the wavelength information of the entire image plane has been obtained.
Accordingly, with the above-described technique, it has been impossible to treat a plurality of types of wavelength information in real time.
Also, the angle of incidence of the diffracted light onto the image plane has been great, so that the quantity of light impinging on the photoelectric detector has been small with a result that a sufficient photoelectric output has not been obtained. On the other hand, when a photographic dry-plate has been used, the expanse of the image by the thickness of the senitive layer has been large to reduce the detection accuracy.
Further, the prior art device has suffered from a disadvantage that the angle of incidence .beta.1 of the diffracted light onto the image plane 4 is a grazing incidence of 80.degree. or more and the length of the image plane is very great. That is, the entrance slit has been disposed on the Rowland circle 1 (a circle whose diameter is a principal radius of curvature R) so as to satisfy r/R cos .alpha.=1, where R is the principal radius of curvature of the concave diffraction grating 3, r is the distance between the entrance slit 2 and the center of the diffraction grating, and .alpha. is the angle of incidence of the principal ray onto the diffraction grating (see FIG. 1).